This invention relates to a thermosetting resin composition having heat resistance of class C (180.degree. C. or higher) and a prepolymer obtained therefrom. Particularly, this invention relates to a thermosetting resin composition suitable as a solventless varnish for impregnation, casting, laminating, molding and adhesion, and a prepolymer obtained therefrom.
With an increasing recent demand for miniaturization and weight-saving of electric machines and devices and their use under severe conditions, improvement of insulating materials in heat resistance has been demanded more and more. For example, varnishes for impregnating coils or casting are required to have a low viscosity with good workability (10 poises or lower at the time of impregnation), to show no voids after cured, and to give products having high mechanical strength at high temperatures. Heretofore, heat resistant insulating materials have been studied in the field of solvent type varnishes usable for enameled wire and laminating materials to develop excellent materials such as polyamide-imides, silicones, polydiphenyl ethers, and the like. Since these materials are usually solid, the use of solvents is necessary when used as varnishes. But the solvent type varnishes have many disadvantages in that they are poor in heat dissipation, low in dielectric strength and adhesive strength, poor in moisture resistance and large in deterioration by heating compared with solventless type varnishes, since the solvent vaporizes to easily produce a large number of voids at the time of curing of the solvent type varnishes. Therefore, it becomes very important for insulating electric machines and devices to use varnishes which are a solventless type containing no solvent and an addition polymerization type (non-polycondensation type) producing no volatile material with a progress of curing reaction.
At present, epoxy resins which have a low viscosity and good workability are used widely as a solventless type varnish relatively excellent in heat resistance. But the upper limit of usable temperature of the epoxy resins is 180.degree. C. even if combined in various ways. Solventless type silicone varnishes have been noticed due to their thermal stability, but their use is limited due to poor mechanical strength at high temperatures and no good solvent resistance.
On the other hand, bismaleimide polymers obtained by reacting an amine with maleic anhydride are not only excellent in heat resistance due to imide groups, but also an addition polymerization product which does not produce volatile components. Therefore, they are noticed recently. But the bismaleimide polymers obtained by radical polymerization are large in thermal shrinkage at the time of curing due to higher cross-linking density. Further, the resin obtained by curing reaction is mechanically brittle and not suitable for practical use. In order to improve such disadvantages, it is proposed to conduct addition polymerization of bismaleimide with a diamine, and the like (U.S. Pat. No. 3,658,764=British Patent No. 1,190,718). The resulting polymer is excellent in physical and chemical heat resistance and electrical properties but cannot be used as a solventless type varnish due to a very high melting point or glass transition point of its resin composition (the viscosity of varnish for coil impregnation being 10 poises or less).
In order to make casting possible without using a solvent, it is proposed to lower the melting point to near room temperature by mixing monomaleimide, bismaleimide, and the like (British Patent No. 1,277,790). But even this process is insufficient in lowering the viscosity and further lowers the heat resistance.
It is also proposed to lower the viscosity by adding an liquid epoxy (Japanese Patent Appln Kokoku (Post-Exam. Publn.) Nos. 20080/74, 1960/74, 9840/75, and 29760/76; and U.S. Pat. No. 3,875,113=British Patent No. 1,412,224). But according to these processes, since precipitates are easily produced at near room temperature, the resulting polymers can hardly be used as solventless type varnishes. Further, there is another disadvantage in that the heat resistance of cured products is largely lowered compared with the case of adding no epoxy.
It is further proposed to mix with a polyvalent carboxylic acid allyl ester such as diallyl phthalate, triallyl trimellitate or triallyl isocyanurate (Japanese Patent Appln Kokoku (Post-Exam. Publn.) No. 13676/78). But according this process, since a principitate is easily produced at near room temperature, the resulting polymer is hardly used as a solventless type varnish. Further, there is another disadvantage in that the cured product is brittle and easily cracked, and low in heat resistance.
Recently, it is also proposed to combine with diallyl bisphenol A, or the like (U.S. Pat. No. 4,100,140=British Patent No. 1,538,470). The resin composition disclosed therein is good in storage stability and is cured with heating. But such a combination makes the melting point or viscosity high, so that the resulting composition cannot be used at room temperature as a resin for casting or a resin for coil impregnation. When heated so as to lower the viscosity (the viscosity of 10 poises or less being necessary for the impregnation), there arises another problem in that the curing reaction proceeds so as to shorten the usable time. Further, the cured product is low in heat resistance.